There has been recently increasing interest in energy storage technology. Electrochemical devices have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Also, in developing such batteries, research on design of a novel electrode and a novel battery has been recently conducted in order to improve capacity density and specific energy.
Among the currently used secondary batteries, lithium secondary batteries, developed in early 1990's, generally have drive voltage and energy density higher than those of conventional batteries using aqueous electrolytes such as Ni—MH batteries, Ni—Cd batteries, H2SO4—Pb batteries, etc., and thus they are spotlighted in the field of secondary batteries.
A lithium secondary battery may be generally manufactured by using a cathode and an anode, which include electrode active materials capable of intercalating/deintercalating lithium ions, and an electrolyte functioning as conduction medium of the lithium ions. Meanwhile, the lithium secondary battery is rechargeable and dischargeable because lithium ions coming out from a cathode active material during a charge process are intercalated into an anode active material, and deintercalated during a discharge process, so that the lithium ions run between both electrodes while serving to transfer energy.
However, in the lithium secondary battery, there is a problem in that during charge/discharge, side reactions occur within the battery by decomposition of a nonaqueous electrolyte solution functioning as an electrolyte, particularly, a carbonate organic solvent, on an electrode surface. Also, when an electrolyte solvent having a large molecular weight, such as ethylene carbonate (EC), propylene carbonate (PC), etc. is co-intercalated between graphite layers of a carbon-based anode, the structure of the anode may be broken down.
It has been known that the above mentioned problems can be solved by a solid electrolyte interface (SEI) layer formed on the anode surface during initial charge of a battery, the SEI layer allowing lithium ions to pass while functioning as a protective layer of the anode surface.
Meanwhile, it is assumed that the SEI layer is formed by reduction of an electrolyte component, a reaction between an electrolyte and a carbon-based anode active material, etc. during initial charge, but lithium ions within a battery irreversibly participate in the formation, thereby reducing the initial capacity of the battery. Accordingly, it is difficult to achieve a high capacity battery.
As an anode active material, a carbon material is mainly used. When a carbon material is used, a high voltage battery can be achieved due to a low potential vs. lithium potential, but it is difficult to achieve a high capacity battery due to a maximum theoretical capacity of only about 370 mAh/g.
As an attempt to achieve a high capacity battery, a method of substituting the carbon electrode active material by a metal or a metalloid-based active material having a high electric capacity, such as Si, has been researched. However, when the metal(loid)-based active material is used, the volume significantly changes according to intercalation/deintercalation of lithium ions, thereby causing problems to be solved, such as cyclability degradation by decomposition of the active material, battery stability degradation by gas generation in a large amount during charge/discharge, etc.